Add Support for BrowserList and Core-JS Polyfills Automatically

[RehanSaeed]

Babel provides the babel-preset-env plugin which uses the [browserlist] syntax shown below to automatically add pollyfills from core-js. This looks at the browsers you want to support and automatically adds the required pollyfills. Babel even goes as far as recommending this approach.

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I never want to have to think about which pollyfills I need to add, TypeScript should provide a typescript-preset-env NPM package to do this for you as it's a very common scenario and easily prone to error. TypeScript should support the browserlist format directly in the tsconfig.json file or in a separate .browserlistrc file.

[mhegazy]

that is what --lib do.. anything specifically you are missing?

[RehanSaeed]

The lib option does not polyfill anything. It simply tells TypeScript to assume that certain API's are available. Babel goes further than the lib option and automatically polyfills too.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[RehanSaeed]

Can this be reopened? This issue is marked as 'Need more info' which I've already supplied. I think I've layed out my proposal clearly.

[levenleven]

Would be nice to have auto polyfilled everything that is specified in --lib and used in code base

[kitsonk]

No it wouldn't... There are many ways to provide functional polyfills and assuming one size fits all would be anti-pattern for TypeScript, as it is a syntactical compiler. In particular it contravenes the following non-goal:

6. Provide additional runtime functionality or libraries. Instead, use TypeScript to describe existing libraries.

Also it touches on this non-goal:

4. Provide an end-to-end build pipeline. Instead, make the system extensible so that external tools can use the compiler for more complex build workflows.

Which is entirely consistent with the approach of the language to this point. TypeScript can be forced to assume some functionality is available (like Promise or Symbol) and use those, but does not provide any functional polyfills.

[sebinsua]

Does the TypeScript team disagree with the approach being taken by babel-preset-env#useBuiltins then?

I think this issue should be re-opened to properly discuss this.

[RehanSaeed]

Babel is TypeScript's major competitor and provides this functionality as a separate package and even recommends using babel-preset-env. If you use Babel, you do not wake up at night in sweats thinking about polyfills.

TypeScript could add this functionality by providing a similar typescript-preset-env NPM package. As it stands, TypeScript developers working in the browser have to get deep into core-js and other pollyfills to get their work done. This is the one area where the grass on the Babel side is far greener.

[stereobooster]

Babel is TypeScript's major competitor

I don't think so. Babel has no type system.

I suppose this is the official solution to the problem https://github.com/Microsoft/TypeScript-Babel-Starter

[RehanSaeed]

@stereobooster It doesn't matter that Babel doesn't have a type system. When a developer decides what front end language to use today, the chances are that they choose between Babel and TypeScript. Other languages come a distant third.

Thanks for the interesting link 🥇 , JS type checking is a half way house, you'll catch some errors but not all.

[Ciantic]

While this might be out of scope of TypeScript, I would like to chime in. I came here because I thought like this:

Since TypeScript knows the types (and calls) it can infer the required polyfills. E.g. if user uses

[1,2,3,4].includes(3)

TypeScript knows this, so it could add includes polyfill. Or mark it as required.

Maybe some other tool could then add all the required polyfills once the TypeScript has collected them.

However, I note that this could be implemented as a tool completely separate from TS.